1. Field of the Invention
This invention generally relates to piezoelectric thin-film resonators and filters, and more particularly, to a piezoelectric thin-film resonator and a filter having the same, in which an upper electrode and a lower electrode are provided to overlap each other and sandwich a piezoelectric thin film therebetween in a region having an elliptical shape.
2. Description of the Related Art
With the rapid spread of high-frequency wireless devices, as represented by mobile telephones, there is more demand for small-sized and lightweight high frequency filters for use in high frequency bands of 900 MHz to 5 GHz. Filters that mainly include surface acoustic wave devices are utilized in the afore-mentioned field. In these days, however, an attention is being focused on a piezoelectric thin-film resonator and a filter having the same, because the device is excellent in high frequency characteristics, can be downsized, and can be fabricated onto a monolithic substrate.
The piezoelectric thin-film resonators include, for example, Film Bulk Acoustic Resonators (FBAR) and Solidly Mounted Resonator (SMR). A FBAR type piezoelectric thin-film resonator (hereinafter, referred to as Conventional Art 1) is disclosed in FIG. 1 and FIG. 2 of Japanese Patent Application Publication No. 60-189307. A lower electrode, a ZnO film that serves as a piezoelectric thin film, and an upper electrode are formed on a silicon substrate having silicon oxide films on both surfaces thereof. Below a region in which the upper electrode and the lower electrode overlap each other, and sandwich the piezoelectric thin film (hereinafter, referred to as membrane region), there is a space arranged in the substrate to include the membrane region. According to Conventional Art 1, a via hole is provided to piece through the substrate. The via hole is formed by wet etching from a bottom surface of the silicon substrate.
Another piezoelectric thin-film resonator (hereinafter, referred to as Conventional Art 2) is disclosed in Japanese Patent Application Publication No. 2003-204239. The via hole is provided below the lower electrode by dry etching the silicon substrate. Yet another piezoelectric thin-film resonator (hereinafter, referred to as Conventional Art 3) is disclosed in FIG. 3 and FIG. 4 of Conventional Art 1 and Japanese Patent Application Publication No. 2000-69594. A cavity is provided below the membrane region, as a space in the substrate, instead of the via hole.
When a high frequency electrical signal is applied between the upper electrode and the lower electrode in the FBAR type piezoelectric thin-film resonator, an elastic wave is excited by the inverse piezoelectric effect in the piezoelectric thin film sandwiched between the upper electrode and the lower electrode in the membrane region. Meanwhile, the distortion made by the elastic wave is converted into the electrical signal by the piezoelectric effect. The elastic wave is wholly reflected by surfaces of the upper electrode and the lower electrode that are respectively in contact with air. A resonance occurs in a frequency that a total film thickness of the upper electrode, the piezoelectric thin film, and the lower electrode equals to an integral multiple of ½ of a wavelength of the elastic wave. The piezoelectric thin-film resonator or the filter having desired frequency characteristics is available by controlling a resonance frequency on the basis of the film thickness. It is preferable that the membrane region is included in the space in the substrate, since the resonance of the elastic wave occurs in the membrane region.
The SMR type piezoelectric thin-film resonator has a structure in which a film having a high acoustic impedance and a film having a low acoustic impedance are alternately deposited with a film thickness of ¼ of the wavelength of the elastic wave, so as to use as an acoustic reflection film instead of the space in the substrate.
As disclosed in Japanese Patent Application Publication No. 2004-312611, there is a piezoelectric thin-film resonator (hereinafter, referred to as Conventional Art 4) that includes a membrane region having a rectangular shape, the lower electrode provided to include the membrane region, and the via hole having a rectangular shape. Also, as disclosed in Japanese Patent Application Publication No. 2002-223144, there is a piezoelectric thin-film resonator (hereinafter, referred to as Conventional Art 5) that includes a membrane region having a rectangular shape, the lower electrode provided to include the membrane region, and the cavity having a rectangular shape.
There exist problems, however, in Conventional Art 1, Conventional Art 2, and Conventional Art 3, in that the resonance characteristics and the filter characteristics vary depending on the variation caused by the fabrication process. In addition, there exist other problems in Conventional Art 4 and Conventional Art 5 in that the resonance characteristics and the filter characteristics are degraded. Furthermore, Conventional Arts 1 through 5 suffer from a common problem of low reliability. Moreover, if the resonance characteristics and the filter characteristics are further improved, resonance characteristics and the filter characteristics vary and the reliability is lowered.